Group a Streptococcus (S. pyogenes or GAS) accounts for an estimated 500,000 deaths per year primarily from complications of rheumatic heart disease, a postinfectious sequel of GAS infection, and from invasive GAS infections. The less severe but far more common syndromes of pharyngitis (streptococcal sore throat) and skin infections cause enormous morbidity and economic losses due to absenteeism from school and work. Ongoing difficulties with treatment failure and recurrent infection likely reflect, in part, the exquisite adaptation of GAS for survival in the human pharynx. GAS is exclusively a human pathogen, and its preferred environmental niche is the human throat. The CsrRS two-component regulatory system, first described by our laboratory, is an important element in GAS adaptation to the host environment. The CsrRS system (also called CovRS) participates in transcriptional regulation of more than 100 genes including those that direct synthesis of key virulence factors such as the hyaluronic acid capsule, streptolysin S, streptokinase, streptodornase, and proteins Mac and GRAB. During the prior funding period, we discovered that CsrRS responds to environmental Mg++: high (10 mM) Mg++ activates CsrRS and represses virulence factor expression, whereas low (1 mM) Mg++ results in derepression. Our recent experiments suggest strongly that CsrRS responds also to the human cathelicidin antimicrobial peptide, LL-37, in a manner opposite to that stimulated by high Mg++. These observations have suggested the novel hypothesis that LL-37 (and/or other antimicrobial peptides) signal through CsrS, thereby alerting GAS to the activation of host defenses and triggering upregulation of bacterial factors that protect the organisms from host effectors. Four Specific Aims will test this hypothesis. We plan to (1) characterize antimicrobial peptide signaling through CsrRS, (2) characterize the molecular interaction of LL-37 and Mg++ with CsrS, (3) reconstitute CsrRS signaling in a cell-free system in vitro, and (4) determine the effects of CsrRS signaling in pathogenesis. Results of these studies will provide new mechanistic insight into an important GAS regulatory system and its role in pathogenesis. PUBLIC HEALTH RELEVANCE: Group A Streptococcus is a major human pathogen as the cause of streptococcal sore throat, skin infection, invasive disease, and rheumatic fever. We plan to study how GAS senses activation of human host defenses during infection through specialized bacterial proteins that bind and respond to human antimicrobial peptides and magnesium ions. We will determine how these interactions signal GAS to change expression of genes that enhance its ability to survive in the human throat and to cause infection. Understanding this process may lead to new strategies for prevention or treatment of GAS infection.